Cold hardening methods that are known include for example the so-called cold box method, in which two components of a synthetic resin system are introduced to the core sand and the sand is hardened as soon as an amine, for example an alkyl amine or a methyl formiate, is added as a catalyst.
One of these components can be for example a polyester resin, a polyether resin or an optional synthetic resin of liquid consistency with reactive hydroxyl groups. The second component is in all cases an organic isocyanate. The two components are basically mixed with the mold sand and then shaped. To catalyze the reaction and the handling and to permit satisfactory use of the amine, various investigations have been undertaken heretofore.
The known methods have, however, a common drawback in that the hardening process requires significant time. For example, the shaping of the core-sand mixture in the shaping tool or mold of a core-shaping machine, often requires only a fraction of a second, contrasting with the subsequent gas treatment for hardening the core which requires several seconds, thereby making the gas treatment a high cost factor.
To reduce the gas-treatment time or the hardening time, attempts have been made to add the amine in excess with the danger that a resolubilization of the binder can occur which can reduce the possible end strength of the core to about 80 to 85%.
In a further process (EP 0229959) metering pumps between the catalyst source and the mixing location for the carrier gas and catalyst are turned on in order to have better metering of the catalyst which also gives rise to an unsatisfactory result since the pressure conditions in the catalyst feed are initially absolutely indifferent with respective metering processes.
It has also been proposed (CH Patent 603276), to store both catalyst vapor/carrier gas mixture and also the compressed air each in a metering vessel and to fire them one after the other impulsively into the core whereby the compressed air with a greater volume is stored and heated to a higher temperature than the catalyst vapor/carrier gas mixture. For this feature, however, the technical cost is enormous and apparatus of this kind has little variability.
In addition, the European patent EP 0881 014 describes a method of the aforedescribed type in which the valve means encompasses a multipath valve in the supply line of the storage vessel which can be temporarily switched to a recycling line running to the supply vessel for pressure equalization in the feed system.
This feature enables the pressure conditions in the catalyst feed for each dosing operation to be maintained constant following the respective preceding pressure equalization.
Present-day techniques can utilize program-controlled switching.